Charisma is only useful for 1 purpose in Fallout NV. Companions. Like fallout 3, charisma really doesn't affect anything since most checks are based on your barter or speach skill level, not your charisma. 1 charisma and 100 speech is the exact same as 10 charisma and 100 speech. Its better to max intelligence for more skill points to get your skills up to high levels.
Now, again the one thing that charisma does affect in NV that it didn't in fallout 3 is your companions armor/attack bonus, which is up to 50% at 10 charisma. This is a huge amount, making tough companions even tougher. This is usually only good if your making a non-combat character and are using your companions as the main meat in fighting. A 50% boosted companion can kick major butt even in very hard mode. So, 1 charisma is good for most games, but if your playing a non-combat character for roleplaying, etc max your charisma and intelligence for easier play.